yoda: Make one option assignment auto-assign something else to others

The "imply <expr>" line for an option (not argument) will lead to every
assignment to the option in question will lead to assigning smth to
other(s) specified by <expr>.

E.g.

option foo
 string foo string

option bar
 bool
 imply foo = foobar

For boolean targets '= value' part may be omitted.

(plan: make 'imply foo = $bar' or smth similar to work)

Signed-off-by: Pavel Emelyanov <xemul@sacred.ru>